Investment casting is the preferred casting method for producing jewelry, dental appliances and other intricate cast products. During investment casting of jewelry, for example, an original metal ring is produced. This metal model is then used to produce a rubber mold. Typically, several layers of rubber are laid down both below and above the model. The rubber is then pressed and vulcanized, creating a cavity around the model. When the model is removed, the cavity remains.
A wax pattern is then formed by injecting wax into the cavity in the rubber mold. Once the mold is filled, the wax is allowed to cool and then the mold is opened to reveal a wax pattern identical to the original ring. To mass produce jewelry, a number of such wax patterns are produced. Each wax pattern is then attached to a central wax stem, creating a tree. This tree is then placed into a flask--typically a metal cylinder with a plurality of holes in its side walls. A sleeve is placed around the flask and "investment"--a powdered substance, similar to plaster mixed with water--is poured around the wax tree. The flask is then subjected to vibration and vacuum to eliminate any air pockets between the investment and the tree.
Next, the flask is heated. When heated, the wax melts and the investment hardens. It is common to produce the stem from a wax with a lower melting point than the wax patterns. Consequently, when heated, the stem melts first, creating a central drain for the wax patterns. After the wax has drained, the hardened investment is a mold capable of producing a set of precious metal rings. To cast gold rings, gold is poured into the investment mold--under vacuum or centrifugal force to force the air out of the mold--and allowed to harden. The mold is then broken open, revealing a gold tree with numerous gold rings attached. The rings are then trimmed from the tree and polished, and the gold stem melted to salvage its gold for later use.
The quality of the precious metal ring depends on the quality of the wax pattern created from the original metal model. Therefore, it is important to control the variables affecting the filling of the rubber mold with wax. One variable is the amount of air in the mold. FIGS. 1a and 1b illustrate a typical mold 10. The mold 10 is essentially two slabs--10a and 10b--of interlocking rubber. The mold can be opened along split 12. Passage 14 leads from a nozzle engagement portion 16 to the ring cavity 18. The cavity 18 has two prongs: 18a and 18b. One prior art method of evacuating air involves applying a vacuum to the stem 14. However, air tends to leak through the side splits 12.
Slits 20 lead from cavity 18 to the sides of the mold. When the wax is injected into the mold, any air remaining in the mold can be forced out through the slits 20. However, the slits also facilitate leakage into the mold when a vacuum is applied. Failure to completely evacuate air from the mold will result in incomplete filling and a useless wax pattern.
When wax is injected into the mold, the pressure tends to force apart the two mold halves. To prevent this, the mold is clamped. However, overclamping will distort the cavity and underclamping will result in "finning" of the wax pattern. Thus, the correct clamping force must be precisely calculated and applied.
Wax quality and temperature also affect proper filling. Prior art wax injectors maintain the wax in a liquid state prior to injection. However, if the wax temperature is too high, the wax can decompose and separate; if too low, the wax can solidify prematurely. If the wax is solid, the valve can be destroyed. Therefore, a need exists for a wax injector that ensures the wax temperature is sufficiently high for injection and yet not high enough to damage the wax.
Air bubbles entrained in the molten wax can ruin the wax patterns. Prior art wax injectors use a single nozzle to inject the wax and to apply the vacuum. In other words, the mold engages the nozzle and a vacuum. A valve then couples the nozzle to a wax reservoir for injection. When the operator switches the nozzle from evacuation to injection mode, air can become entrained in the wax and make the wax pattern unusable.
A final source of wax pattern defects is entrained water vapor. Air--usually obtained from the shop compressed air system--pressurizes the wax pot. This high-pressure air usually contains considerable moisture that combines with the hot liquid wax. Upon injection, the water vapor may depressurize and expand in the wax pattern, ruining the pattern. Thus, a need exists for a wax injector that reduces or eliminates the water entrained in the wax.